Conventionally, a chloroprene rubber (CR)-based rubber composition has been employed for the outer cover rubber of hydraulic hoses, as the composition is well-balanced in a wide range of rubber characteristics, such as weather resistance, wear resistance, heat resistance, oil resistance, and low temperature properties, which are important factors in the outer cover rubber.
On the other hand, in recent years, environmental considerations call for an increasing demand for rubber compositions of a blend system of acrylonitrile-butadiene rubber (NBR) and ethylene-propylene-diene rubber (EPDM), without using chloroprene rubber (CR) containing chlorine, where acrylonitrile-butadiene rubber (NBR) is to develop oil resistance and ethylene-propylene-diene rubber (EPDM) is to develop weather resistance and heat resistance (see PTL 1 and PTL 2 below). In the above-mentioned blend system, sulfur, a cobalt salt of an organic acid, silica, and a resorcinol-formaldehyde (RH)-based resin are used as compounding agents for securing adhesiveness to wires.